Portable communications devices operating in two or more radio systems have become common in recent years. If such a communications device functions only in one system at a time, it is usually equipped with an antenna that has two operating bands or one band which is wide enough to cover both bands used by the two systems, for example. Two separate antennas may be used if the communications device can function simultaneously in two systems, especially if the frequency bands of the systems are relatively close to one another. With separate antennas, the mutual interference of the systems can be made smaller than with a common antenna. However, the mutual interference is not completely removed because there exists a certain electromagnetic coupling between the antennas. This problem can be in principle alleviated by increasing the distance between the antennas, which, however, will in practice make the structure too large. An interfering transmitter may also be equipped with an antenna filter the attenuation of which increases steeply on that side of the pass band where the operating band of the affected receiver is located. The order of such a filter is high, resulting in higher production costs and problems related to the pass-band attenuation of the filter. All increases in losses between the power amplifier and antenna will result in increased current consumption in the power amplifier and potential heating problems in the device.
Electromagnetic coupling between antennas can also be reduced by arranging electrical isolation between them. FIG. 1 illustrates such a known solution. FIG. 1 shows the antenna end of a transmitter operating according to a first system, and the antenna end of a receiver operating according to a second system. The transmitter includes a series connection of a RF power amplifier PA, transmitting end antenna filter SFI and a transmitting antenna 110. The filter SFI is relatively simple in that its pass-band attenuation is not harmfully high. The receiver includes a receiving antenna 120 which is connected to a receiving end antenna filter RFI which in turn is connected to a low-noise amplifier LNA. The first system is for example GSM1800 (Global System for Mobile Communications) and the second system e.g. GPS (Global Positioning System) in which the receiving frequency is 1575.42 MHz. In that case GPS reception will be susceptible to interference from GSM transmissions because the gap between the GPS receiving frequency and GSM transmission band is only 135 MHz. In FIG. 1 there is a line 105 between the antenna symbols, referring to an arrangement which electromagnetically isolates the transmitting and receiving antennas. Such an arrangement may be e.g. a grounded metal strip placed between the antenna elements. A disadvantage of this solution is that it increases the amount of hardware as well as production costs. Moreover, the directional characteristics of the antennas may suffer.
An object of the invention is to reduce said disadvantages associated with the prior art. An antenna structure according to the invention is characterized by that which is specified in the independent claim 1. A radio device according to the invention is characterized by that which is specified in the independent claim 13. Some advantageous embodiments of the invention are specified in the other claims.